In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units (UE) such as mobile telephones (“cellular” telephones) and laptops with wireless capability, e.g., mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB”, “B node”, or (in LTE) eNodeB. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of radio access networks, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). An entity known as the Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). Another name used for E-UTRAN is the Long Term Evolution (LTE) Radio Access Network (RAN). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are performed by the radio base stations nodes. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
The evolved UTRAN (E-UTRAN) comprises evolved base station nodes, e.g., evolved NodeBs or eNBs, providing evolved UTRA user-plane and control-plane protocol terminations toward the wireless terminal. The eNB hosts the following functions (among other functions not listed): (1) functions for radio resource management (e.g., radio bearer control, radio admission control), connection mobility control, dynamic resource allocation (scheduling); (2) mobility management entity (MME) including, e.g., distribution of paging message to the eNBs; and (3) User Plane Entity (UPE), including IP Header Compression and encryption of user data streams; termination of U-plane packets for paging reasons, and switching of U-plane for support of UE mobility. The eNB hosts the PHYsical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. The eNodeB also offers Radio Resource Control (RRC) functionality corresponding to the control plane. The eNodeB performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers.
The Long Term Evolution (LTE) standard is based on multi-carrier based radio access schemes such as Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and SC-FDMA in the uplink. The Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
There are two basic types of frame structures for LTE. The first type of frame structure pertains to Frequency Division Duplex (FDD). The second type of frame structure (the more pertinent of the two types to the technology disclosed herein) pertains to Time Division Duplex (TDD) and is described, e.g., in 3GPP TS 36.211 V8.5.0 (2008-12) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8), incorporated by reference herein in its entirety. The second type of LTE frame structure comprises ten subframes. As shown in Table 4.2-2 of in 3GPP TS 36.211 V8.5.0 (2008-12) section 4.2) the second type of LTE frame structure can have six differing uplink-downlink configurations (with differing numbers of downlink (DL) subframes, uplink (UL) subframes, and special subframes).
In the time domain, each LTE subframe (having 1 ms duration) is divided into two slots, each slot being 0.5 ms in duration. The transmitted signal in each slot is described by a resource grid of subcarriers and symbols. Each element in the resource grid is called a resource element (RE) and is uniquely defined by an index pair (k,l) in a slot (where k and l are the indices in the frequency and time domain, respectively). In other words, one symbol on one sub-carrier is a resource element (RE). Each symbol thus comprises a number of sub-carriers in the frequency domain, depending on the channel bandwidth and configuration. See, e.g., 3GPP TS 36.211 V8.5.0 (2008-12) section 5.2.
In Long Term Evolution (LTE) no dedicated data channels are used, instead shared channel resources are used in both downlink and uplink. These shared resources, e.g., the downlink shared channel (DL-SCH) and the uplink shared channel (UL-SCH) are each controlled by one or more schedulers that assign(s) different parts of the downlink and uplink shared channels to different UEs for reception and transmission respectively.
The assignments for the downlink shared channel (DL-SCH) and the uplink shared channel (UL-SCH) are transmitted in a control region covering a few OFDM symbols in the beginning of each downlink subframe. Each assignment for downlink shared channel (DL-SCH) or uplink shared channel (UL-SCH) is transmitted on a physical channel named the Physical Downlink Control Channel (PDCCH) in the control region. A Physical Downlink Control Channel (PDCCH) is mapped to (e.g., comprises) a number of control channel element (CCEs). Each control channel element (CCE) consists of thirty six resource elements (REs). The total number of available control channel element (CCEs) in a subframe will vary depending on several parameters like number of OFDM symbols used for PDCCH, number of antennas, system bandwidth, Physical HARQ Indicator Channel (PHICH) size, etc.
There are three basic physical channels and a reference signal (RS) on the LTE uplink. An uplink physical channel corresponds to a set of resource elements which carry information from higher layers. The LTE uplink includes the following three physical channels: the Physical Uplink Shared Channel (PUSCH); the Physical Uplink Control Channel (PUCCH); and the Physical Random Access Channel (PRACH). The uplink physical signal is used by the physical layer but does not carry information originating from higher layers.
The Physical Uplink Control Channel (PUCCH) carries uplink control information and supports multiple formats. The supported formats and their respective modulation schemes are described in Table 5.4-1 of section 5.4 of 3GPP TS 36.211 V8.5.0 (2008-12). Among the formats here of primary interest are format 1a (having a BPSK modulation scheme) and format 1b (having a QPSK modulation scheme). The modulation schemes of format 1a and format 1b are also described with reference to Table 5.4.1-1 of section 5.4 of 3GPP TS 36.211 V8.5.0 (2008-12), which is reproduced as Table 1 below.
TABLE 1PUCCH formatb(0), . . . , b(Mbit − 1)d(01a 01 1−11b00101−j10J11−1
As alluded to previously, one property of Time Division Duplex (TDD) is that the number of UL and DL subframes can be different. In the configurations where there are more DL subframes than UL subframes, multiple DL subframes are associated with one single UL subframe for the transmission of the corresponding control signal. For example, for each dynamically scheduled transmission in the DL subframes, acknowledgement/negative acknowledgement (ACK/NACK) bits need to be transmitted in an associated UL subframe to support proper Automatic Repeat Request (ARQ) or Hybrid Automatic Repeat Request (HARQ) operation. For example, if a wireless terminal is scheduled in a multiple of DL subframes which are all associated with one UL subframe, the wireless terminal needs to transmit multiple ACK/NACK bits in that UL subframe.
Thus, in LTE TDD any asymmetry in the downlink (DL)/uplink (UL) configuration results in multiple simultaneous ACK/NACK feedbacks for one wireless terminal (UE) on the uplink control channel (PUCCH) in case of heavy downlink (DL) transmission. Table 2 shows the number of DL subframes associated with each UL subframe for each of the aforementioned six DL/UL TDD configurations.
TABLE 2Subframe index n0123456789DL:UL0101101configurations12121244332244459611111
The cells in Table 2 which have no numbers are understood to be associated with downlink subframes. Except for configuration 5, up to four DL subframes are associated with one UL subframe, which needs up to eight ACK/NACK feedbacks in case of MIMO on PUCCH (since there are two antenna elements for the downlink (DL) for MIMO). For configuration 5, up to as many as eighteen ACK/NACK feedbacks may be necessary on PUCCH. Having so many ACK/NACKs can result in a big payload for PUCCH to carry and further limits PUCCH coverage.
In view of the effect of the taxing effect of a potentially large number of ACK/NACKs being carried on PUCCH, a scheme of ACK/NACK “bundling” has been proposed. TSG-RAN WB1 #52, R1-081110, ‘Multiple ACK/NAK for TDD’, Ericsson, Motorola, Nokia, Nokia Siemens Networks, Qualcomm. Typically in such bundling ACK/NAKs from one or several DL subframes are combined (“bundled”) by performing a logical AND operation of all ACK/NACKs to result in a single ACK/NAK report. Yet ACK/NACK bundling can cause unnecessary retransmission, since all packets in the bundling window must be retransmitted if there is even one packet received incorrectly. Consequently, adopting ACK/NACK bundling across a system typically reduces downlink (DL) throughput. See, e.g., 3GPP TSG RAN WB1 #53, R1-081988, ‘Support of Multiple ACK/NAK Transmission in TDD’, Texas Instruments.
In some situations it is desired to obtain a tradeoff between PUCCH coverage and downlink (DL) efficiency by, e.g., supporting multiple ACK/NACK with some kinds of methods on reducing payload. For example, some scenarios performing a bundling operation only in the spatial domain or the temporal domain (but not both). These scenarios can provide more information for eNodeB to do flexible scheduling. For example, when only spatial bundling is adopted for each downlink (DL) subframe, there are only two ACK/NACKs in the case of two DL subframes; three ACK/NACKs in the case of three DL subframes; and four ACK/NACKs in case of four DL subframes. In addition, for configuration 5 of Table 2, some form of bundling can be applied to further limit the payload, e.g. four ACK/NACK feedbacks.
Currently there are two main proposals for supporting the transmission of up to four ACK/NACK with targeting to make a tradeoff between ACK/NACK bundling and the actual number of ACK/NACK feedbacks. These two proposed solutions are respectively referred to herein as the “resource reserved solution” and the “channel coding solution”.
The resource reserved solution is based on PUCCH format 1a/1b and is described, e.g., in 3GPP TSG RAN WB1 #53, R1-081988, ‘Support of Multiple ACK/NAK Transmission in TDD’, Texas Instruments [incorporated herein by reference]. In the resource reserved solution, multiple PUCCH resources should be reserved in terms of the number of downlink (DL) subframes to provide multiple PUCCH channels to be selected. Multiple ACK/NACK bits are jointly coded to select a BPSK/QPSK symbol and a corresponding PUCCH channel as the transmission structure of format 1a or format 1b. The reserved PUCCH resources are derived from mutliple DL subframes, or from multiple CCEs in one DL subframe.
There are several drawbacks of the resource reserved solution. A first drawback is that more PUCCH resources should be reserved when the number of downlink (DL) subframes increases, e.g. four PUCCH resources are needed in case of four downlink (DL) subframes. A second drawback is that the reserved PUCCH resources maybe vary each time and depend on downlink (DL) subframes or the CCE in each DL subframe. In other words, the solution needs and involves more downlink (DL) information as the mapping rule, especially with the increasing of DL subframes. Further, possibly part of the information will be lost if some DL assignments are missing, which results in confusion on the association between DL CCE and PUCCH resources. A third drawback is that correct detection depends on the joint detection of both BPSK/QPSK symbols and the used PUCCH channel. A fourth drawback is that the eNodeB will do blind detection among all reserved PUCCH channels.
The channel coding solution together with discontinuous transmission (DTX) is based on format 2 and is described in TSG-RAN WG1 #53, R1-082001, ‘On Multiple ACK/NAK for LTE TDD’, Ericsson [incorporated herein by reference]. In the channel coding solution, in order to reuse existing PUCCH format 2, Reed Muller codes are also used for the coding of multiple ACK/NACK, the same as CQI reporting. There are also several drawbacks to the channel coding solution. A first drawback is that this solution requires a PUCCH format 2 resource, and hence higher layer configuration similar to CQI reporting, which will result in different transmission structure for different TDD configurations, such as one or two ACK/NACK (format 1a/1b) and more than two ACK/NACK (format 2). Another drawback occurs, e.g., in the case of small payload, in which some Reed Muller codes may not be optimized codes (e.g., simplex codes are superior to Reed Muller codes at rate 2/20, for example).
For two downlink (DL) subframes, the resource reserved solution is a better solutation than the channel coding solution (since, e.g., the Reed Muller code [20,2] is not a optimized code). For four downlink (DL) subframes, the two solutions have very close performance.
What is needed, therefore, and an object of the technology disclosed herein, are method and apparatus for providing ACK/NACK on the PUCCH in a manner that balances PUCCH coverage and downlink (DL) efficiency.